Addicting is an interesting choice of words, as one might interpret it as not just your favorite but like what you just can't control yourself from, such as getting stuck in your head or singing in public places and then wanting to die... not that that has ever happened <*cough*>. Hypothetically, REALLY hypothetically speaking, it's possible I could see the following choices to list a few:

Tenjou Tenge's OP was literally what sparked my renewed interest in anime last year. I'd seen the opening as a trailer years ago but never went and watched the show until a few months ago when I heard the song again.